Artificial dielectric using interspersed rods



5 ;4 L l 3, 7 a? M. 2,

ARTIFICIAL DIELECTRIC USING INTERSPERSED RODS Filed July 5, 1963 2Sheets-Sheet 1 FIG.

y 1, 1966 P. w. HANNAN 3,254,345

ARTIFICIAL DIELECTRIC USING INTERSPERSED RODS Filed July 5, 1963 2Sheets-Sheet 2 INDEX OF REFRACTION l I I I FREQUENCY (KMC) INDEX OFREFRACTION L00 2 a l E l fi' 0 .l .2 .3 .4 .5 .6 .7 .8 .85 .9

LENGTH OF METALLIC PORTION 22-23 N FIG UNIT LENGTH 22 FIG. 4

United States Patent 3,254,345 ARTIFICIAL DIELECIlIlIC gJSINGINTERSPERSED 0D Peter W. Hannah, Northport, N.Y., asignor to HazeltineResearch Inc., a corporation of Illinois Filed July 5, 1963, Ser. No.292,915 7 Claims. (Cl. 343-911) This invention pertains to artificialdielectrics and more particularly to artificial dielectrics composed ofthree sets of interspersed rods. Each rod may be all dielectric or allmetal or may comprise a series of metal sections separated by gaps ofdielectric material for example.

In a microwave antenna employing a lens it is desirable to minimize theweight and expense involved in a dielectric material. It may also benecessary to provide a controlled variation of the refractive index withposition in the lens. An artificial dielectric composed of metal ordielectric elements can achieve these results.

Unfortunately, the electrical properties of an artificial dielectric arelikely to be poor compared with those of a natural dielectric. Some ofthese defects are anisotropy (different refractive indexes for some wavedirections and polarizations), random errors in refractive index (causedby random errors in packing the elements) and perhaps also excessivereflection (in a metal loaded delay medium having a wave impedance lowerthan that of a natural dielectric with the same refraction index).

The objects of this invention are to provide artificial dielectricswhich avoid defects of prior artificial dielectrics, which have improvedelectrical properties, and which at the same time are practical toconstruct both structurally and economically.

In accordance with the invention, an artificial dielectric comprises afirst set of rods, whose properties vary along their length, arranged ina spaced array with all rods substantially parallel to each other, asecond set of rods, whose properties vary along their length, regularlyinterspersed with but spaced from the rods of the first set and arrangedso that all rods of this second set are substantially parallel to eachother and substantially perpendicular to the rods of the first set, athird set of rods, whose properties vary along their length, regularlyinterspersed with but spaced from the rods of the first and second setsand arranged so that all rods of this third set are substantiallyparallel to each other and substantially perpendicular to the rods ofboth the first and second sets and means for supporting all these rodsin the resulting interspersed, spaced, three dimensional array; wherebythere is formed an artificial dialectric Whose characteristics ofrefractive index relative to position in the volume of artificialdielectric are determined by the rod construction.

For a better understanding of the present invention, together with otherand further objects thereof, reference is bad to the followingdescription taken in connection with the accompanying drawings, and itsscope will be pointed out in the appended claims.

In the drawings, FIG. 1 represents a small portion of an artificialdielectric constructed in accordance with the invention; FIG. 2 shows asingle rod, for use in artificial dielectrics in accordance with theinvention, whose dimensions vary along its length; and FIGS. 3 and 4 arecurves useful in describing the operation of the invention.

The basic structure involves three sets of rods or cylinders oriented inthree orthogonal directions, each set being interspersed with equalspacing from the other sets. Each rod may for example, be constructedentirely of dielectric material or entirely of metal or it may be madeup of a series of metal sections separated by gaps of air or dielectricmaterial. It should be understood that the rods or cylinders need not becircular in cross section, but may be of rectangular, elliptical or anyother desired cross section. T

Referring now to FIG. 1. There is shown a portion of an artificialdielectric employing metal sections separated by gaps of dielectricmaterial. The artificial dielectric shown in FIG. 1 includes a first setof rods which will be called set X, including rods X1 to X9 inclusive.Also included in the FIG. 1 arrangement is a second set of rods whichwill be called set Y, including rods Y1 to Y9, inclusive. Furtherincluded in FIG. 1 are a third set of rods which will be called set Z,including rods Z1 to Z9, inclusive. FIG. 1 finally includes means forsupporting all three sets of rods in the resulting three dimensionalarray. In the illustrated configuration, these last means can beconsidered as taking the form of six sheets of thin transparent phenolicmaterial, one appearing at each face of the cube shown in FIG. 1. InFIG. 1 the lines 20 are indicative of the edges of one of such sheets.Only one of such sheets is shown so as to indicate that actually a muchlarger volume of artificial dielectric may be utilized. In otherarrangements the rods may be supported in the spaced relation by a lowdielectric constant Examining FIG. 1 in greater detail, it will be seenthat all rods of set X are substantially parallel to each other. Also,all rods of set Y are regularly interspersed with but spaced from therods of set X and the rods of set Y are arranged so that all of the Yrods are substantially parallel to each other and also substantiallyperpendicular to the rods of the X set. Also, the rods of set 2 areregularly interspersed with but spaced from the rods of both set X and Yand the rods of set 2 are arranged so that all these Z rods aresubstantially parallel to each other and substantially perpendicular tothe rods of both sets X and Y. Thus, there results an interspersed,spaced three dimensional array of rods forming an artificial dielectricin accordance with the invention.

In operation, currents flow along the metal sections of those rods whichare parallel to the electric field of an incident electromagnetic wave,creating a concentration of electric field in the appropriate gapsseparating the metal sections of the rods. This results in the reducedwave velocity (refractive index greater than 1) that is usually desiredin an artificial dielectric. Since the rods are equally dispersed inthree orthogonal directions, this effect is isotropic provided thespacing of the metal elements is rnuch less than a wav elength. Forexample, the isotropic property would be essential in the case of aspherical Luneberglens for an antenna operating in all directiomhermore,the dimensions of each gap do not depend on accidental factors as in arandom packed artificial dielectric for example, but instead can beprecisely measured and controlled. As a result, the refractive index canachieve a remarkable freedom from random errors, as is essential in anylens antenna intended to have a narrow beam.

Referring now to FIG. 2, there is shown a single rod of a different typethan the rods included in the artificial dielectric shown in FIG. 1. Itwill be seen that the dimensions of the rod shown in FIG. 2 vary alongits length. As shown, the dimension 30 (corresponding to 22 in FIG. 1)remains constant over the length of the rod. However, the gaps 31-36grow progressively smaller from left to right. The physical constructionof the rod shown in FIG. 2 may be considered similar to that of the rodsin FIG. 1 except that the FIG. 2 rod is shown as utilizing a hollowdielectric cylinder rather than a solid cylinder as in FIG. 1. Usingrods similar to the one shown in FIG. 2, artificial dielectrics can beconstructed which have a refractive index which varies relative toposition within the volume of artificial dielectric involved. The

dimensions of each rod in such an artificial dielectric would bedetermined by the particular variation of refractive index desired. Manydifferent variations of dimensions along the length of rods are possibleand FIG. 2 illustrates just one example.

In this metal section type of artificial dielectric, the rods areseparated from each other and the metal elements occupy a relativelysmall portion of the volume; consequently, blocking of the magneticfield of the wave is small. As a result, the wave impedance and surfacereflection is not appreciably worse than that of a natural dielectric.This property would also be important in any application involvingdiscontinuous media, such as an ordinary lens or a stepped Luneberglens. The interspersed geometry of the rods facilitates constructionsince each rod is physically independent of the others and there are nocontact problems. Where light weight is essential, the metal elementscan be thin-walled, hollow cylinders or strips, or small diameter solidcylinders supported by dielectric sections. Alternatively, the rods maycomprise dielectric rods or hollow cylindegs with pieces of metal foilwrapped therefdn to for'm the metallic sections. The basic configurationis susceptible to modifications which simplify the production of largeamounts of artificial dielectrics. In applications requiring very largevolumes of artificial dielectrics, economic considerations are, ofcourse, very important.

The all-dielectric rod type of artificial dielectric is useful where asimple, high power, nonresonant material is desired and where weight andcost are less important. The all-metal rod type is an advance ratherthan a delay medium and is useful for special applications. In theinterspersed rod configuration, both of these types retain the featuresof isotropic and precisely controllable refractive index. In view of theextensive work which has been done in the filed of artificialdielectrics the design of any of the aforementioned types of artificialdielectrics, in accordance with the invention, can be carried out bypersons skilled in this art, once they have the present concepts clearlyin mind and no discussion of the design principles is necessary here.

With reference to FIG. 1, the dimensions of an artificial dielectricactually constructed are as follows:

' v The metal sections were formed by pieces of aluminum foil .00075inch thick, such as 26, wrapped around solid polystyrene rods and thegaps, such as 28, were merely uncovered portions of the polystyrenerods.

Tests on the artificial dielectric having the dimensions just listedindicated that at or below one kilomegacycle the index of refraction wasapproximately 1.3 for all angles of incidence and all polarizations. Theactual curves including these results are shown in FIG. 3. The curves A,B and C in FIG. 3 represented the characteristics measure with ray pathdirections as indicated by the arrows A, B, and C, respectively, shownrelative to an outline of the artificial dielectric shown in FIG. 1.

Referring now to FIG. 4, there is shown a curve indicating the widerangeof refractive indexes available by varying the dimension 22relative to dimension 23. Thus, it will be appreciated that artificialdielectrics constructed in accordance with the invention, will possesscharacteristics of refractive index which are determined by the rodconstruction and spacings.

While there have been described what are at present considered to be thepreferred embodiments of this invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention, and it is, therefore,aimed to cover all such changes and modifications as fall within thetrue spirit and scope of the invention.

What is claimed is:

1. An artificial dielectric comprising:

a first set of rods, whose properties vary along their length, arrangedin a spaced array with all rods substantially parallel to each other;

a second set of rods, whose properties vary along their length,regularly interspersed with, but spaced from, the rods of said first setand arranged so that all rods of this second set are substantiallyparallel to each other and substantially perpendicular to the rods ofsaid first set;

a third set of rods, whose properties vary along their length, regularlyinterspersed with, but spaced from, the rods of said first and secondsets and arranged so that all rods of this third set are substantiallyparallel to each other and substantially perpendicular to the rods ofboth said first and second sets;

and means for supporting all said rods in the resulting interspersed,spaced, three-dimensional array;

whereby there is formed an artificial dielectric whose characteristicsof refractive index relative to position in the volume of artificialdielectric are determined by the rod construction.

2. An artificial dielectric comprising:

a first set of rods arranged in a spaced array with all rodssubstantially parallel to each other, said rods comprising sections ofconductive material separated by sections of dielectric material;

a second set of rods regularly interspersed with, but spaced from, therods of said first set and arranged so that all rods of this second setare substantially parallel to each other but perpendicular to the rodsof said first set, said rods comprising sections of conductive materialseparated by sections of dielectric material;

a third set of rods regularly interspersed with, but spaced from, therods of said first and second sets and arranged so that all rods of thisthird set are substantially parallel to each other but perpendicular tothe rods of both said first and second sets, said rods comprisingsections of conductive material separated by sections of dielectricmaterial;

and means for supporting all said rods in the resulting interspersed,spaced, three-dimensional array.

3. An artificial dielectric in accordance with claim 2, wherein each rodis of substantially identical construction and comprises a cylinder ofdielectric material covered at spaced intervals by encircling bands ofconductive material;

and all rods are uniformly and repetitively spaced so that the resultingartificial dielectric has a substantially uniform refractive indexthroughout its volume.

4. An artificial dielectric in accordance with claim 2, wherein each rodis of substantially identical construction and comprises a hollowcylinder of dielectric material covered at spaced intervals byencircling bands of conductive material;

and all rods are uniformly and repetitively spaced so that the resultingartificial dielectric has a substantially uniform refractive indexthroughout its volume.

5. An artificial dielectric comprising:

a first set of rods, whose properties vary along their length, arrangedin a spaced array with all rods substantially parallel to each other,said rods comprising sections of conductive material separated bysections of dielectric material;

a second set of rods, whose properties vary along their length,regularly interspersed with, but spaced from, the rods of said first setand arranged so that all ductive material providing gaps of exposedalong the length of the rod;

dielectric 2o MAN and all rods are uniformly that the resultingartificial diele and repetitively spaced so ctric has a substantiallyuniform refractive index throughout its volume.

7. An artificial dielectric in accordance with claim 6,

wherein each cylinder of dielectric material is hollow.

References Cited by the Examiner UNITED STATES PATENTS 2,936,453 5/1960Coleman 3439l0 3,055,005 9/1962 Siegel 343-911 3,165,750 1/1965 Tell343-311 OTHER REFERENCES Brown: Artificial Dielectrics, Proceedings ofThe I.E.E.

(London), vol. 100, No. 5, October relied o R. F. HUNT, JR., Assistant1953, pages 51-62 KARL SAALBACH, Primary Examiner.

Examiner.

1. AN AIRTIFICAL DIELECTRIC COMPRISING: A FIRST SET OF RODS, WHOSEPROPERTIES VARY ALONG THEIR LENGTH, ARRANGED IN A SPACED ARRAY WITH ALLRODS SUBSTANTIALLY PARALLEL TO EACH OTHER; A SECOND SET OF RODS, WHOSEPROPERTIES VARY ALONG THEIR LENGTH, REGULARLY INTERSPERSED WITH, BUTSPACED FROM, THE RODS OF SAID FIRST SET AND ARRANGED SO THAT ALL RODS OFTHIS SECOND SET ARE SUBSTANTIALLY PARALLEL TO EACH OTHER ANDSUBSTANTIALLY PERPENDICUALR TO THE RODS OF SAID FIRST SET; A THIRD SETOF RODS, WHOSE PROPERTIES VARY ALONG THEIR LENGTH, REGUALRLYINTERSPERSED WITH, BUT SPACED FROM, THE RODS OF SAID FIRST AND SECONDSETS AND ARRANGED SO THAT ALL RODS OF THIS THIRD SET ARE SUBSTANTIALLYPARALLEL TO EACH OTHER AND SUBSTANTIALLY PERPENDICULAR TO THE RODS OFBOTH SAID FIRST AND SECOND SETS; AND MEANS FOR SUPPORTING ALL SAID RODSIN THE RESULTING INTERSPERSED, SPACED, THREE-DIMENSIONAL ARRAY; WHEREBYTHERE IS FORMED AN ARTIFICAL DIELECTRIC WHOSE CHARACTERISTICS OFREFRACTIVE INDEX RELATIVE TO POSITION IN THE VOLUME OF ARTIFICIALDIELECTRIC ARE DETERMINED BY THE ROD CONSTRUCTION.